Mixing device and method

Abstract: Mixing material (M) around inner agitating means (20) in a mixing vessel (10) is urged upward and outward by rotating the inner agitating means (20) in one direction, and simultane-ously, mixing material (M) around outer agitating means (30) is urged downward and inward by rotating the outer agitating means (30) in the opposite direction, consequently to cause the mixing materials urged upward and downward to be circulated by convection in the mixing vessel (10) and the mixing materials urged outward and inward to collide between the inner and outer agitating means (20,30), thus forming a high-pressure region (T) between the inner and outer agitating means (20,30). The mixing materials (M) are mashed in the high-pressure region (T) and well mixed in a short time with a high efficiency without being agglutinated to the inner agitating means (20).

Description: This invention relates to a mixing device and method for effectively stirring or mixing different materials such as raw materials for the manufacture of a variety of concrete. Mixing devices such as a concrete mixer for mixing cement, water and aggregate to produce cement paste or ready-mixed concrete are commonly divided into three types. One of them is a vessel-rotating type mixer (so-called vessel-tilting type mixer) comprising a rotary vessel and inner mixing blades fixed inside the mixing vessel. In this vessel-rotating type mixer, the rotary vessel is rotated to mix different materials by utilization of free fall of the materials in the rotating vessel. Thus, this mixer making use of gravitation is inferior in efficiency. A second type is a so-called pan-type mixer having a stationary pan-like vessel and a mixing paddle disposed on the axial centre of the vessel. A third type is a horizontal-paddle type mixer having a stationary vessel and one or more rotary mixing paddles horizontally supported in the vessel. The aforementioned vessel-fixed type mixers are now finding widespread acceptance for actual use. In the concrete mixer of this type, however, mixing materials are mixed with a shearing force produced by rotating the paddles, and therefore, cannot be satisfactorily circulated by convection in the vessel and well mixed in a short time. There has been a conventional concrete mixer of the vessel-fixed type for mixing powder material such as flour and granular medicines, as shown in FIG. 1. This prior art concrete mixer has a single agitating spiral screw 2 vertically sugported in a vessel 4, and a draft cylinder 6 arranged coaxially around the screw 2 so as to circulate the mixing material in the vessel by convection. That is, by rotating the screw 2, the mixing material M in the vessel 4 is caused to move downward inside the draft cylinder 6 and upward outside the draft cylinder 6. In the case of dealing with sticky mixing materials in the concrete mixer using such a draft cylinder around the screw, however, the mixing material M near the screw 2 and the inner surface of the vessel 4 tends to be prevented from moving and is stagnated around the regions as indicated by the symbols T1 and T2 in FIG. 1, consequently to obtain an mixture insufficiently mixed. Furthermore, the situation that the sticky mixing material within the draft cylinder 6 is agglutinated to the screw 2 while being mixed becomes a matter of great concern. As a result, the screw 2 is rotated in sympathy with the mixing material, thus bringing about a so-called "racing" phenomenon in which the mixing material in the vessel is not circulated by convection, thus causing the mixer to malfunction practically. The racing phenomenon conspicuously occurs when mixing the mixing material having high viscosity. Also in a dual-screw type mixer having two screws arranged in parallel within a mixing vessel, mixing material admitted in the vessel is apt to lose fluidity around the inner surface of the vessel and be stagnant. Under certain circumstances, the mixing material is possibly agglutinated to the screws, consequently bringing about the racing phenomenon causing the mixer to malfunction practically. Thus, the conventional concrete mixers of any type have a common disadvantage such that they cannot uniformly stir or mix different mixing materials with a high efficiency and are apt to give rise to the racing phenomenon in which the mixing materials are agglutinated to and rotated together with the rotating screw or screws. Considering the case where cement paste for lightweight concrete or aerated concrete by way of example, the insufficient mixing as noted above entails a problem of bringing forth small cement bubbles in the cement paste. The cement bubbles in the cement paste result in defects on the micron order in hardened concrete, thus lowering the strength of the concrete. Although the mixing materials should be sufficiently mixed to obviate such problems and obtain concrete products of high quality, it has been desired to enhance the efficiency of production of the cement paste so as to produce cement paste in a short period of time in large quantities in view of productivity. US-A-1,796,659 discloses a method and device for mixing material having the features of the preambles of Claims 1 and 4. This invention is made to eliminate the drawbacks suffered by the conventional mixing device as described above and has an object to provide a mixing device and method capable of swiftly stirring or mixing different kinds of materials with a high efficiency without causing the mixing materials to be agglutinated to agitating means or stagnated in a mixing vessel to produce a high quality mixture in which the raw materials are uniformly dispersed. Another object of the invention is to provide a mixing device having inner and outer agitating means coaxially arranged and means for effectively driving the agitating means in opposite directions in order to swiftly and uniformly stirring or mixing various kinds of materials with a high efficiency. The invention provides a method for mixing material in accordance with Claim 1. Furthermore, the present invention provides a mixing device in accordance with Claim 4. The mixing material around the rotating inner agitating means is urged upward and outward, and the mixing material around the rotating outer agitating means is urged downward and inward. The mixing materials thus urged centrifugally and centripetally come in collision with each other at the middle portion between the inner and outer agitating means to form a high-pressure region thereat. In the high pressure region, the particles of the mixing materials undergo shearing friction which positively exerts a mashing action on the mixing materials. As a result, a well-blended mixture having no particle bubbles can be obtained. The inner agitating means is preferably formed of a screw having a spiral blade inclined in one direction. By rotating the screw in one direction, the mixing material around the screw acquires upward and centrifugal propulsive forces. The outer agitating means preferably includes agitator vanes each having a forward-bent upper portion and an oblique lower portion. By rotating the agitator vanes along the inner surface of the vessel in the direction opposite to the direction in which the inner agitating means rotates, the mixing material around the orbit of the agitator vanes along the inner surface of the vessel acquires lower and centripetal propulsive forces. In addition, the outer agitating means may be provided with rectifier plates which revolve together with the outer agitating means to heighten the effect of circulating the material in the vessel by convection. Thus, by rotating the inner and outer agitating means in opposite directions at a time, the materials at the central portion and peripheral portion of the vessel are urged toward the middle portion thereof and come into collision with each other, resulting in formation of the aformentioned high-pressure region. Owing to the high-pressure region, the mixing material are no longer agglutinated to the inner or outer agitating means. Therefore, according to the mixing device of the invention, the different kinds of mixing materials can effectively be mixed in a short time, and a well-blended high-quality mixture can be produced with a high efficiency. FIG. 1 is a schematic side view showing a prior art mixing device; FIG. 2 is a schematic perspective view showing one embodiment of the mixing device of this invention; FIG. 3 is a schematic side section showing the device of FIG. 2; FIG. 4 is a schematic plan view of FIG. 2; FIG. 5 is a conceptual sketch showing the state in which mixing materials are circulated by convection in the mixing device of the invention; and FIG. 6 is a schematic side view of another embodiment of the mixing device according to this invention. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only. The mixing device of this invention is very useful for stirring or mixing different materials to obtain a high quality mixture with a high efficiency. The mixing device and method of this invention will be described hereinafter on the assumption that raw materials for concrete products, including cement, aggregate and water, are used as the mixing material by way of example. However, any kind of materials may be dealt with. The mixing device shown in FIGS. 2 through 4 as one embodiment comprises a substantially cylindrical mixing vessel 10 into which mixing materials are admitted, inner agitating means 20 rotatably arranged vertically at the center of the mixing vessel 10, and outer agitating means 30 arranged rotatably along the inner surface of the circumferential wall of the mixing vessel 10. The mixing vessel 10 assumes a generally cylindrical shape comprising a substantially cylindrical upper part 10a having an upper opening, and an inverted truncated cone shaped lower part 10b. The upper opening of the vessel 10 is covered with a lid member 12 having an axial hole 12a and a material inlet surrounded by a hopper 12b. The mixing vessel 10 has an outlet 14 and a gutter 16 for discharging a mixture resultantly produced in the vessel. The inner agitating means 20 has a screw 22 comprising a rotary shaft 22a arranged vertically through the axial hole 12a in the lid member 12, and a spiral blade 22b spirally wound around the rotary shaft 22a. The spiral blade 22b in this embodiment turns round the rotary shaft 22a leftward like a right hand screw. Therefore, by rotating the screw 22 rightward (in the direction indicated by the arrow R1 in FIG. 3), the mixing material M around the screw 22 in the mixing vessel 10 is urged upward. At the time the mixing material M is stirred and urged upward by rotating the screw 22, it is incidentally urged centrifugally. Consequently, the material M in the vessel 10 flows upward as indicated by the arrow f1 in FIG. 5 and simultaneously in the centrifugal direction as indicated by the arrow f2. Although the screw 22 in this embodiment is formed like a right hand screw, it may be formed like a left hand screw as a countermeasure. In the case of the left hand screw, the screw 22 may be rotated in the reverse direction to urge the mixing material upward. The inner agitating means 20 is driven by driving means 26 including an electric motor, which is mounted on the top of the rotary shaft 22a. The outer agitating means 30 comprises rotary units 32 which revolve along the inner surface of the vessel 10 to urge the mixing material in the vessel 10 in the downward and centripetal directions, and rectifier plates 34 attached to each rotary unit 32 so as to revolve along with the rotary units 32 to urge centripetally the material in the vessel 10. The rotary unit 32 is formed so as to cause the mixing material M in the vessel 10 to flow in the direction opposite to that in which the mixing material around the screw 22 is urged by the inner agitating means 20. That is, the inner agitating means 20 causes the mixing material M to flow upward (f1) and centrifugally (f2), and at the same time, the outer agitating means 30 causes the mixing material to flow downward (f3) and centripetally (f4) as shown in FIG. 5. The rotary unit 32 in this embodiment comprises a cylindrical rotary shaft 321 coaxially arranged around the rotary shaft 22a of the screw 22, a pair of rotary arms 322 horizontally extending radially from the rotary shaft 321, supporting rode 323 vertically extending from the rotary arms 322, and agitator vanes 324 held by the supporting rods 323. The agitator vane 324 is formed of a plate having an upper portion 324a bent forward relative to the direction in which the rotary unit 32 revolves, a middle portion 324b at which the agitator vane is attached to the supporting rods 323, and an oblique lower portion 324c which is inclined relative to the radial direction so as to urge the material in the vessel 10 centripetally when rotating the agitator vane. The rectifier plate 34 is disposed in a space between the screw 22 and the agitator vane 324 so as to impart a centripetal motion to the mixing material in the vessel. The outer agitating means 30 is driven by acquiring rotation from driving means 36 including the electric motor through transmission means 37a and 37b such as gears. Although the driving means 26 for the inner agitating means 20 and the driving means 36 for the outer agitating means 30 respectively have the separate electric motors as illustrated in FIG. 1, both the driving means may be operated by a single electric motor. Next, the operation of the aforementioned mixing device of this invention will be explained. The mixing material M admitted in the mixing vessel 10 is stirred by rotating the inner agitating means 20 and the outer agitating means 30 in the opposite directions in such a state that the material around the rotating screw 22 is urged upward (f1) and centrifugally (f2), and simultaneously, the material around the rotary unit 32 is urged downward (f3) and centripetally (f4) as shown in FIG. 5. The centripetal force exerted on the mixing material is increased by the rotating rectifier plate 34. Thus, the mixing material M is circulated by convection in the vessel 10, giving rise to a convection current of the material. At the same time, the material forcibly moving outwardly from the central portion comes into collision with the mixing material forcibly moving inwardly from the peripheral portion in the vessel, consequently forming a substantially annular high-pressure region T at the middle portion between the central portion and the peripheral portion as shown in FIG. 5. In this high-pressure region T, the mixing material M undergoes strong shearing friction repeatedly, and is intensely mashed to cause the particles of the mixing material to be intimately merged together. Moreover, repulsion brought about as reaction force of collision of the mixing materials in the high-pressure region T acts on the material held by the spiral blade 22b of the screw 22, thereby to heighten fluidity of the mixing material. As a result, the mixing material tending to be stagnant in the spiral blade 22b is forced upward, and therefore, prevented from being agglutinated to and rotated together with the screw 22. Besides, since the rotary units 32 revolve along the inner surface and bottom of the cylindrical vessel 10, the mixing material M is positively moved all over the inside of the vessel 10 to be circulated by convection, resulting in production of a well-blended high-quality mixture. Since the inner agitating means and the outer agitating means are operated at different speeds in accordance with the quality and property of the material to be mixed, the most suitable mixing condition can be established. The inventors of this invention produced some mixing devices according to the present invention by way of trial and carried on experiments to confirm the superior performance of the mixing device of this invention. Comparative experiments were made using the mixing device of this invention, which comprises a mixing vessel of 650 mm in diameter and 750 mm in height, and a screw having an outer diameter of 240 mm, a spiral blade of 140 mm in width and a diametral pitch of 1:1. As a comparative conventional mixer, a forced two-axle type concrete mixer having two agitating screws and a vessel having the substantially same volume as the mixing device of this invention was used. Three sets of raw materials for concrete admitted in the respective vessels of the mixer of this invention and the comparative mixer in these experiments are shown in Table 1 below. That is, comparative raw materials #CS1∼#CS3 were mixed by the comparative conventional mixer, and sample raw materials #ES1∼ES3 were mixed by the mixing device of this invention. Portland cement was used in the experiments. Sample Design Slump (mm) Water/Cement Ratio (%) Raw Materials [Unit Volume (kg/m)] Cement Water Fine Aggregate Coarse Aggregate #CS1 18 58 314 182 823 954 #CS2 12 58 300 174 820 988 #CS3 8 58 279 162 826 1034 #ES1 18 58 309 179 828 959 #ES2 12 58 297 172 823 994 #ES3 8 58 276 160 823 1039 In the experiments, the mixer of this invention was operated by rotating the screw 22 at 300 rpm, and simultaneously, the rotary unit 32 at 30 rpm in opposite directions to mix the designated raw materials for concrete. On the other hand, the conventional mixer was operated by rotating the screws at 45 rpm. Each set of the raw materials was mixed continuously until its design slump was obtained. The times required to obtain cement paste samples having the design slump values for the respective raw materials were determined in advance after repeating tests. Namely, it was found that the conventional mixer requires 60 seconds to obtain the design slumps, and the mixer of this invention requires 15 seconds to obtain the same design slumps. The comparative experiments were conducted by using the cement paste samples produced by mixing the designated mixing materials for the prescribed times, respectively. The characteristics of the resultant cement paste samples (compressive strength of hardened concrete after specified days) are shown in Table 2 below. Sample Mixing Time (sec) Slump (cm) Compressive Strength (kgf/cm) Age (7 days) Age (28 days) #CS1 60 17.5 250 370 #CS2 13.0 242 377 #CS3 8.0 253 385 #ES1 15 17.5 251 386 #ES2 11.5 255 389 #ES3 8.0 258 399 It is clear from the results of the experiments as shown in Table 2 that the cement paste produced by mixing the designated materials for only 15 seconds with the mixer of this invention has strength equal to or higher than that produced by mixing the substantially same materials for 60 seconds with the conventional concrete mixer. Furthermore, cement paste was produced by mixing raw materials for concrete with a foaming agent in order to back up the excellent characteristics of the cement paste produced by the mixer of the present invention. As a result, the cement paste having a great number of minute bubbles uniformly dispersed therein could be obtained in a short time and turned into high-quality foamed lightweight concrete which is not permeably by water. Still more, the mixing raw materials were mixed with the foaming agent and a high-performance water reducing agent. As a result, cement paste in which innumerable microscopic bubbles of the order of ten-odd µm in size are dispersed uniformly could be obtained. In another experiment, toughened staple fibers of about 14 µm in diameter and about 6 mm in length were added to the raw materials and mixed by the mixing device of this invention. It was confirmed that the toughened staple fibers are uniformly dispersed by a ratio of more than 1% in one liter of the cement paste resultantly obtained in spite of the staple fiber being about 428 in aspect ratio. After forming and curing the cement paste thus obtained, fiber-reinforced superduty concrete in which the toughened staple fibers are uniformly dispersed could be produced. When the raw materials for concrete were mixed with the foaming agent, water reducing agent, and staple fibers, high-quality cement paste possessing the respective excellent characteristics of these additives could be obtained. Thus, by use of the mixing device according to the present invention, even materials which are frequently either impossible or very difficult to mix with the conventional mixing devices can be easily mixed with a high efficiency, consequently to produce cement paste of high quality. Although the aforementioned mixing device of the invention has two rotary units 32 each extending horizontally from the cylindrical rotary shaft 321, three or more rotary units may be adopted insofar as the entire balance of the rotary unit 32 can be kept. FIG. 6 shows a modified embodiment in which additional rotary units 432 having the same structure as the rotary units 32 may be attached to the rotary arm 322 in place of the rectifier plate 34 used in the foregoing embodiment. By rotating the rotary units 32 together with the additional rotary units 432 in the direction R2 opposite to the direction R1 in which the screw 22 rotates, the shearing friction brought about by the collision of the mixing materials at the high-pressure region between the inner and outer agitating means can be increased to enhance the mixing efficiency. Incidentally, the constituent elements such as the screw, agitator vanes and rectifier plates may be made of not only metal, but also plastic, ceramic or any other hard materials. As is apparent from the foregoing description, according to the mixing device of the present invention, since the mixing materials admitted in the mixing vessel can be effectively circulated by convection in the vessel while causing collisions among the particles of the materials, a high-quality mixture, in which the particles are uniformly dispersed can be produced in large quantities in a short time with a high efficiency. Since the repulsion of the collisions among the particles of the mixing materials at the high-pressure region in the vessel is exerted to the materials around the screw, the materials are circulated by convection in the vessel without being agglutinated to the screw and stagnated in the spiral blade of the screw. Besides, the collisions among the particles of the mixing materials give rise to strong shearing friction among the particles of the materials, thereby intensely mashing bubbles of particles in the mixture. In particular, the mixing device of the invention is adapted for producing cement paste for lightweight concrete, and can mix any kinds of materials because the inner and outer agitating means can be separately driven at different speeds in accordance with the quality and property of the mixing materials.

Claims: A method for mixing materials (M) admitted in a mixing vessel (10) with inner and outer agitating means (20,32) arranged coaxially, which comprises rotating said inner and outer agitating means (20,32) in opposite directions so as to urge the materials (M) in opposite directions, characterized in that said inner and outer agitating means (20,32) are rotated at different speeds and are shaped to cause the materials (M) to collide in the radial direction so as to form a high-pressure region (T) between said inner and outer agitating means, and in that further agitating means (34,432) disposed coaxially between said inner and outer agitating means in said high-pressure region (T), are rotated with said outer agitating means (32) so as to enhance the mixing efficiency. A method according to Claim 1, wherein said mixing materials (M) includes cement, water, fine aggregate and coarse aggregate. A method according to Claim 2, wherein said mixing materials (M) includes at least one selected from foaming agent, water reducing agent, and staple fibres. A mixing device comprising a mixing vessel (10), inner and outer agitating means (20,32) coaxially arranged in said mixing vessel (10) and drive means (26,36) for rotating said inner and outer agitating means (20,32) in opposite directions, characterized in that said inner and outer agitating means (20,32) are rotated, in use, at different speeds and are shaped to cause the materials (M) to collide in the radial direction so as to form a high-pressure region (T) between said inner and outer agitating means and in that further agitating means (34,432) are disposed coaxially between said inner and outer agitating means in said high-pressure region (T) for rotation with said outer agitating means (32) so as to enhance the mixing efficiency. A mixing device according to Claim 4, wherein said inner agitating means (20) includes a screw (22) comprising a rotary shaft (22a), and a blade (22b) spirally wound around said rotary shaft. A mixing device according to Claim 5, wherein said outer agitating means comprises a rotary unit (32) comprises a cylindrical rotary shaft (231) coaxially arranged around said rotary shaft of said screw (22), a pair of rotary arms (322) horizontally extending radially from said rotary shaft (321), supporting rods (323) vertically extending from said rotary arms, and agitator vanes (324) held by the supporting rods. A mixing device according to Claim 6, wherein said agitator vane (324) comprises an upper portion bent (324a) forward relative to the direction in which said rotary unit revolves, a mounting middle portion (324b) at which said agitator vane (324) is attached to said supporting rods, and an oblique lower portion (324c) which is inclined relative to the radial direction so as to urge the mixing material (M) in said vessel (10) centripetally when rotating said agitator vane (324). A mixing device according to Claim 6 or Claim 7, wherein said further agitating means comprise an additional rotary unit (432) substantially similar to said rotary unit (32) constituting said outer agitating means, and which is attached to said rotary unit (32). A mixing device according to any one of Claims 4 - 7, wherein said further agitating means comprise rectifier plates (34) fixed to said outer agitating means (30) so as to rotate coaxially to said inner agitating means (20). A mixing device according to any one of Claims 4 - 9 comprising a substantially cylindrical mixing vessel (10) for containing mixing materials (M), having a central portion and a peripheral portion therein, said inner agitating means (20) being vertically supported at the central portion of said mixing vessel (10), and said outer agitating means (32) being arranged in said mixing vessel (10) so as to rotate along said peripheral portion of said mixing vessel coaxially with said inner agitating means (20) in the opposite direction thereto.

Inventors: INOUE TAKAO, SAITO MAKOTO, INOUE, TAKAO, SAITO, MAKOTO
Applicants: KAJIMA CORP, KAJIMA CORPORATION
Application year: 1994
Main classification: B01F7
IPC codes: B01F7